Cathode materials for lithium-ion batteries typically include layered compounds “LiMO2,” spinel compounds “LiM2O4,” and olivine compounds “LiMPO4.” These layered and spinel compositions are some of the most widely used in lithium ion batteries.
Typically, in the structure of a layered compound having the general formula “LiMO2,” the oxygen anions form a close-packed fcc lattice and cations occupy the 6-coordinated octahedral crystal sites. In this configuration, the “MO2” slabs and “Li” layers are alternately stacked.
Within the structure of a spinel compound having the general formula “LiM2O4,” the metal cations generally occupy the octahedral sites, but ¼ of them are located in the “Li” layer, which means that ¼ of the sites in the transition metal layer are vacant. Li+ ions occupy the tetrahedral sites in the Li layer that share 5 faces with the empty octahedral sites in the transition metal layer. Together the three-dimensional MO2 host and the vacancies in the transition metal layer ensure that there are three-dimensional diffusion pathways for the lithium ions. When used in a lithium ion battery, spinel material generally has better rate capability than a layered oxide material, but this benefit is balanced by a lower capacity of the spinel material. This discrepancy, as well as the spinel material's capacity degradation at elevated temperature, is typically a drawback to the use of spinel material as a cathode material.
Accordingly, there is an ongoing need for new cathode materials for use in lithium ion batteries. Particularly, there is a need for additional cathode materials with increased thermal stability and good rate performance.